The long-term objective of my laboratory is to achieve a detailed understanding of the molecular mechanism(s) by which cell fates are specified via cell-cell interactions. In C. elegans embryos, Wnt signaling induces the E blastomere to produce endoderm. Without this induction, E adopts the fate of its sister, MS. Because POP-1 represses 2 known E-specific genes in MS, and POP-1 level in E is lowered by Wnt signal, it has been proposed that Wnt signal promotes endoderm fate by downregulating POP-1 activity in E. Our recent results force a reevaluation of this model. We show that POP-1 has a positive role in the expression of E-specific, Wnt-responsive genes in E, and a negative role for them in MS. In addition, we have identified POP-1-dependent, Wnt non-responsive genes in MS. The overall goal of this proposal is to determine how POP-1 activates E-specific genes in response to Wnt signaling but represses them in the absence of Wnt signaling, and how POP-1 can promote the expression of MS-specific genes but repress E-specific genes, all in the same cell. Specific Aims for this proposal are: (1) Genetic dissection for factors regulating POP-1 transcriptional activity in the E blastomere. We will revisit established genetic interactions using the expression of E-specific and MS-specific genes as readouts. We will also identify positive regulators of E-specific gene expression by way of a genetic screen. (2) To determine the effect of phosphorylation by Wnt signaling on POP-1 transcriptional regulation of E-specific genes. We will determine the domain(s) and residues of POP-1 phosphorylated upon Wnt signaling important for its ability to activate E-specific genes. (3) To investigate the mechanism by which phosphorylation of POP-1 can alter POP-1 transcriptional activity. We will investigate the effect of POP-1 phosphorylation in E on interactions of POP-1 with corepressors and coactivators and with DNA. (4) To characterize transcriptional activation of MS-specific genes by POP-1. POP-1 must regulate E-specific and MS-specific genes differently. We will analyze how POP-1 regulates target genes in MS, and will examine the role of POP-1 characteristics shown to be important for E-specific gene activation on MS-specific gene expression. Deregulation of the Wnt signaling pathway has been found in various cancers. It is our hope that understanding how Wnt regulates the expression of target genes via TCF proteins could someday help target identification for anticancer drug design.